With File Sharing enabled in Administration -> Server -> Settings, each channel will present a simple Files panel where you can drag in a file, and give it a name plus some notes. If the file is an image or simple video file, a thumbnail will be generated and it will appear as so:
Choose a file to view it in the browser (if it is a compatible media file), or use the buttons on the right to direct download, copy a direct public link or delete the file. There is currently a 256MB file limit, configurable up to 1GB at the time of installation by setting an environment variable 'UPLOAD_MB_LIMIT' (e.g. UPLOAD_MB_LIMIT=512).